1. Field of Invention
The present invention relates to starting powder for producing sintered parts which consists of an Al--Si based alloy powder exhibiting low thermal expansion and high ductility. The sintered parts mentioned above can be used for office machines and machines related to computers.
The present invention also relates to a method for producing sintered parts and sintered aluminum alloy.
2. Description of Related Arts
It has recently become necessary in the field of office machines and computer-related machines to reduce electric power consumption and to prevent noise generated by machine vibration. Improvement of the portability of such machines is also necessary. In order to meet such requirements, light weight aluminum alloys are increasingly being used for the parts of such machines. The demand is for aluminum alloys with a low coefficient of thermal expansion such that there is no mismatch of the machine parts even under environmental temperature changes.
It is an object of the present invention to provide an inexpensive method for producing the Al--Si based alloy parts which can be used for the applications as described above and which exhibit a low coefficient of thermal expansion.
Heretofore, die casting was the conventional method for producing the complicated parts of an Al--Si based alloy with a low coefficient of thermal expansion. Die casting is advantageous in the point that three-dimensionally complicated shapes can be produced. On the other hand, the dimension accuracy of the die cast products is insufficient. In addition, since the die cast articles must have a taper for removing the same from a mold, they are not infrequently subjected after the casting to expensive machining. Furthermore, the reliablity of the die cast products is not sufficient because cast defects, such as blow holes, degrade the properties.
According to another method employed for producing the Al--Si based alloy parts with a low coefficient of thermal expansion, an ingot is produced by melting and is used as the starting material. It is subjected to working to obtain the wrought product. The wrought product, which is blank material, is subjected to machining, such as lathing. However, the Si content of the Al--Si based alloy to be subjected to the above working is approximately 17% at the highest, because segregation is likely to occur in the ingot during casting, and, further, coarse, primary Si crystals precipitate with increase in the Si content, thereby decreasing the workability of the alloy. In addition, a low yield of the working is one of the factors leading to enhanced price of the parts.
Attempts have been made to apply a powder metallurgical method for the production of Al--Si based alloy parts so as to utilize an advantage of such method, i.e., production of near-net-shape, and, hence to eliminate the disadvantages of the die cast or wrought products. The ordinary sintering method involves compacting the powder in a metal die into a near-net shape and then sintering the obtained green compact. The sintering method is therefore a simple process which allows the near-net-shape to be obtained. The sintering method is therefore greatly advantageous from the viewpoint of cost.
However, the Al--Si based alloy is hard and exhibits poor compressibility and compactibility. The green compact therefore cannot be highly densified. In addition, since the Al--Si alloy has a low melting point, the sintering temperature cannot be made sufficiently high for satisfactorily promoting the sintering. It was, therefore, heretofore impossible to obtain by the sintering method parts exhibiting satisfactory mechanical properties, particularly good elongation.
Japanese Unexamined Patent Publication No. 53-128512 filed by the present applicant discloses the following sintering method.
(1) Al--Si alloy powder with Si content of from 10 to 35% by weight is annealed. PA1 (2) The annealed Al--Si alloy powder is mixed with one or more of the following powders so as to obtain a composition consisting of 0.2-4.0% of Cu, 0.2-2.0% of Mg, 10.0-35% of Si, the balance being Al. PA1 (3) Al powder may be further mixed with (2). PA1 (4) The mixture is compacted and then sintered in inert atmosphere. PA1 (a) Mg powder PA1 (b) Al--Mg powder PA1 (c) Al--Cu powder PA1 (d) Al--Mg--Si powder PA1 (e) Al--Cu--Si powder PA1 (f) Al--Mg--Cu powder PA1 (g) Al--Mg--Cu--Si powder PA1 (h) Mg--Cu powder PA1 (i) Mg--Cu--Si powder
(a) Cu powder PA2 (b) Mg powder PA2 (c) Al--Cu alloy powder PA2 (d) Al--Mg alloy powder PA2 (e) Cu--Mg alloy powder PA2 (f) Al--Cu--Mg alloy powder PA2 (g) Cu--Mg--Si alloy powder PA2 (h) Al--Cu--Mg--Si alloy powder
The present inventors made experiments of the method of Japanese Unexamined Patent Publication No. 53-128512 and discovered that notwithstanding fairly good strength properties the ductility was not satisfactory.
The ductility is an important index of the material, related to its reliability. Since the conventional sintered, high Si-Al alloy exhibits poor toughness and hence low ductility, it cannot be occasionally used for parts subjected to relatively high load, such as reciprocating arm-parts.
Attempts have been made to utilize the so-called, powder forging method, so as to improve the mechanical properties of the Al--Si alloy sintered products. That is, the powder-compacting and sintering method is carried out to produce a preform, which is then hot die-forged. However, since the preform is hot-forged, it is likely to stick on the die, and the life of the die is shortened. In addition, it is difficult to finish the hot-forging with the high dimensional accuracy that is required for the parts of an office machine or the like. Therefore, final machining of the hot-forged product is inevitably required in order to enhance the dimensional accuracy.
Proposals have been made to: press-form the powder of Al--Si based alloy and hot-extrude the resultant billet.
Since extrusion of the billet is carried out at under a hot working condition, its plastic deformation is sufficient to rupturing the oxide layer on the particles of the Al--Si based alloy powder. As a result, the particles are brought into contact with each other via the metal surfaces, and the properties of Al--Si based alloy are enhanced (c.f. for example, "All of Aluminum Powder Metallurgy" (text of meeting for publishing the research and development efforts of aluminum powder metallurgy, the meeting being held by the Researching Association of Aluminum Powder-Metallurgy Technique), and "Recent Powder Metallurgy Technique of Aluminum Alloys" (30th Symposium of The Institute of Light Metals). The hot-extrusion process is, however, expensive. In addition, the product of the hot-extrusion process is an intermediate product, which must be further forged or machined to obtain the final shape of parts. The forging or machining lowers the yield and enhances the cost too greatly for the products to be used practically.